Add build instructions (Fixes #23)

BUILD.md

@@ -0,0 +1,8 @@
+## Building from source
+
+* Download a Go release (Go 1.6 minimum): https://golang.org/dl/ or use your system's release if >= 1.6
+* Create a build directory: `mkdir -p ~/go`
+* Point GOPATH environment variable to this directory: `export GOPATH=~/go`
+* Clone the source: `go get github.com/mariadb-corporation/replication-manager`
+* Compile and install: `go install github.com/mariadb-corporation/replication-manager`
+* Add the go binaries directory to your path: `export PATH=$PATH:~/go/bin`